1. Field of the Invention
The present invention relates to apparatus for elevating fluids through a flow path. More specifically, the present invention relates to a system or apparatus for surface controlled gas lifting of petroleum fluids from a subterranean formation through a well conduit to the earth's surface.
2. Brief Description of the Prior Art
In conventional gas lift systems, petroleum fluids are artificially lifted to the surface of a well by injecting a gas under pressure into the column containing the fluid to be elevated. By this means, a desired flowing bottomhole pressure is created in the well which permits elevation of the petroleum fluids to the well head at a desired rate. In a typical continuous flow gas lift system, gas is introduced into the well casing and injected into the petroleum fluid column contained within the tubing string. The casing gas is injected into the tubing through a single gas lift valve referred to as the operating valve which is generally disposed in the tubing string below a series of vertically spaced unloading valves.
When tubing pressure operated (fluid operated) gas lift valves are employed to unload the well, each unloading valve closes automatically when the tubing pressure opposite the valve decreases below a predetermined minimum valve. By this means, each succeeding lower valve is closed as the fluid level in the annulus is successively lowered and the tubing fluid level is raised until the operating or bottommost injection valve is exposed to the injection gas in the casing. The latter valve, the operating valve, is thereafter employed as the single point injection valve for a continuous flow gas lift system. Where casing pressure operated rather than fluid operated valves are employed to unload, closure of each succeedingly lower casing pressure operated valve generally necessitates a succeeding reduction in the casing pressure. For this reason, a lower effective injection gas pressure is available for injection into the tubing at the operating valve as compared with that available with the use of fluid operated unloading valves.
In conventional systems, the regulation range, i.e., the injection gas pressure range over which the operating valve remains open, also presents problems in that the regulation range of a casing pressure operated valve is generally relatively limited resulting in little or no control over the rate of gas injection into the tubing. It is well known that as the well conditions change or as production rates are altered to conform to restrictions imposed by regulatory agencies, the most effective or efficient rate of gas injection may be different from that existing in the well at the beginning of the gas lift operation and it is often desirable to alter the injection rate with such changing conditions or restrictions. Where casing pressure operated unloading valves are used, raising the gas pressure in an attempt to increase the injection rate may only serve to move the point of gas injection up the well to a higher gas lift valve. This is usually undesirable since optimum producing conditions normally require injection at the lowermost valve possible, thus making it clear that the lifting efficiency is reduced whenever the point of injection moves above a specified injection point.
While controlled variation in the rate of introducing gas into the production fluid column is highly desirable, it will be understood that it is impractical to attempt to control the injection rate by replacing the operating valve each time a new injection rate is desired. In short, the prior art apparatus and methods available for gas lift operations have lacked suitable means for effecting surface controlled regulation of the rate at which gas is injected into the fluid column through the operating valve. Moreover, the conditions normally imposed by conventionally employed casing pressure operated unloading valves limit the operating range of prior art systems to pressure levels significantly below the injection gas pressure available at the well head.